1. Field of the Description
The present invention relates, in general, to optical devices used in three dimensional (3D) displays, and, more particularly, to methods and systems for calibrating optical devices or assemblies that have one or more caustic surfaces.
2. Relevant Background
Optical devices with caustic surfaces are used in a wide range of applications such as in glasses-free 3D displays, in directional lighting assemblies, to provide unique toys, and for art installations. In optics, a caustic surface is a surface to which all light rays emanating from a single point and reflected by a curved surface (such as a concave mirror) are tangent, and a caustic may refer to an envelope of light rays reflected or refracted by a curved surface or object or the projection of that envelop of light rays onto another surface. In other words, caustic surfaces may display light from a light source (e.g., a projector) as concentrated patches of light or bright edges.
The most well-known caustic surface is a lens or lens array that may be used in a 3D display. For example, a fly's eye display lenslet may be used in many glasses-free 3D display technologies. In another example, a lenticular array may be used to display a flip image with one or more flipping regions where differing images are displayed to an observer. Glasses-free 3D display technologies may achieve a 3D effect by spatially multiplexing different images corresponding to different viewpoints into different directions by putting a lenslet array on top of an image surface. When the image surface is at the focal plane of the lenslet array, each of the display pixels below one lens is exposed to a different viewing angle, and this effect can be used to show different images to different viewers (e.g., flip images) or even different views of the same scene to achieve a 3D effect. Similarly, interestingly shaped caustic surfaces can be found in many toys, figurines, and gadgets/products, and, in their operation to provide displays or light effects, these may rely on the same principles of sending different image positions in different directions.
The quality of the imagery provided by many of these optical assemblies or devices may be unacceptable or at least diminished because most of these devices are not calibrated at all. The caustic surface (or optical assembly with one or more caustic surfaces) is designed and manufactured, but the alignment of the caustic surface facing toward the display surface(s) can only be done with machine precision. However, imperfections and flaws in the caustic surface due to the manufacturing process as well as slight misalignments, such as due to temperature changes, can lead to distracting or even unacceptable visual artifacts in the displayed images on the display surface(s).
When calibration is performed, it often still does not address issues involved with use of caustic surfaces in display systems. For example, in the case of lenslet arrays (which provide the caustic surface (or the reflective/refractive surface) of the optical assembly or optics), the current state-of-the-art calibration technique relies on special frequency patterns that are used to estimate the lens pitch, which in turn is then used to compute the pattern to display. However, this calibration method fails when lens pitch varies over the lens array and also does not consider other lens parameters including imperfect or changing focal length.
For some 3D display systems, some calibration approaches have been investigated that use a closed feedback loop to expose different images to different viewers. In one exemplary calibration attempt, left and right halves of viewer faces were illuminated with light of different wavelengths and captured by a camera focusing on this audience. The captured information was then adjusted and deformed to illuminate a display surface through an optical system such as by making some content visible only to the illuminated left eyes and some only visible to the right eyes. While being useful for some applications, such systems still require careful alignment and additional calibration of different components, which may be difficult. Further, these systems are often relatively complex to assemble and operate, are expensive to fabricate or assembly, and are typically very bulky.
Hence, there remains a need for improved methods of calibrating optics or optical assemblies that include one or more caustic surface. Preferably, such methods may be implemented with a calibration system that is inexpensive and not overly complex to implement and use. In some cases, it may be preferable for the optical calibration method to output a mapping or calibration map that can be readily used to project light or display images through the caustic surface(s) of the optical assembly onto a display surface(s), e.g., provide a mapping from image pixels to viewing positions.